• closes @ureg.wraps decorator #141, closes expects decorator #143, closes Draft: add wraps_ufunc decorator #203
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This is a reimplementation of #143 (and #203), based on inspect.signature objects to deal with the classification of args and kwargs.

Once complete, we'll be able to define functions like this:

from pint_xarray import expects, unit_registry as ureg


@expects(None, "m", ureg.s, {None: "kg", "lat": "rad"}, return_value={None: None})
def func(a, b, c, d):
    ...

and have the decorator convert any arguments to the appropriate units, or raise an error if the unit spec does not match the argument (either because the units are incompatible, or because the spec requires an xarray object but the argument is a scalar / plain array / quantity).